Telephone trunking system.



WITNESSES M. SCHWARTZ.

TELEPHONE THUNKING SYSTEM- MPLICATION FILED AR. 3| lalli.

Ptenta Feb. 27,1917.

. M. SCHWARTZ.

TELEPHONE TRUNKING sYsLTEm. APPLICATION FILED ma. 3. I913. 1 1,217,510. Patented Feb. 27,1917.

- 3 SJHEETS-SHEET 2- mvmisq aa I I V I IVI/EA/TOE. Q i w M. SCHWARTZ. TELEPHONE munxmasysnm; u

- Paitented Feb. 27,1917.

APPLICATIO N FILED MAR. 3| I913.

. aw Q M MR? v Wromvzrq ENT OFFICE.

MICHAEL scnwaa'rz, or CHIOAGO,- itimoxs, ASSIGNOR 'ro AUTOM TIC ELECTRIC COMPANY, 01 cmcneo, iLL-mozs, A conronmrmn or ILLINQIS.

TELEPHONE 'i'nUN'KmG SYSTEM.

To all whom-it mdy concern:

lie it known that I; Miclunn, Scmw-urrz,

iv citizen of the l nited States of America,

and resident of Ch cagm-in'the county of 5 (look and State of Illinois, have invented certain new aml'uset'ul Improvements in Telephone 'l runking Systems,-ot which the following a specification.

My invention relates to improvements in. automatic telephone systems, and more pan ticularly to an improved :n-rangement'of the switches. and trunks extendin g between these switches, whereby more efficient and efi'ective service can be given. It further relates to an improved construction'of certain oi the switches whereby interference is prevented.

My invention" relates more specifically to a telephone system employing line switches .20 which select an idle one of a group of trunks. which line switches are divided into groups. Het'tofore it -has been customary to have each of these. groups of trunks individual to a articular group of line switches. A more cient arrangement; however. and one which has been found gives just as eliicient service. is to have certain of these trunks individual to'each group of line switches. while otherof the trunks are common to two or more groups. With such an m-range ment it. of course, becomes necessary topro .vide means for preventing a switch ot'oue group seizing-a connnon trunk whi h is al-' ready occupied by a switch of another group. Further specific features of my invention are the provision of a master switch for controlling ,each group of lineswitches. and the provision of means whereby the master switches which control groups of line. 401switches whichlmve access to the same trunk line operate in syncln'onism. by naumeans the two or more .master switches t'hns' ii i i'zitcil in synchronism will always stand with their plungers opposite the. same trunk line. Means are likewiseprovi dcd whereby these master switches op-' crate in symfln'ouism only when they are working over the common'trunks, but operate independently of each other whenthey '50 are operating on the individual trunks. 4 These and other objects of my in\"entiqn willbe more readily understood by reference to the accompanying drawings, in whlch.

l ignre 1, 2 and 3 taken together i'epre sent a .complete clrclntconnectlon between in yeach gioup, are-connected to the same Specification 01! Letters Patent. Patented Feb 27, 1917 Application filed Ketch 3,

1913. serial No. 751,882.

a calling substation A (Fig. 1) and a called substationt-v (Fig.3)- i a Fig. l is a diagran'imaticsketch of the ar rangement ot'trunks leading from the line switches to the first selectors.

The telephone. system shown herein is of A the salue type as that shown and described in British patentto A. E. Keith No. l-i'll of 1910, except that no secondary trunking switches are employed between the subscribers individual switches and' the first selectors. In this system the connection b tween the calling and called subscribers is established through the medium of a preliminary trunk selecting switch B, first lector (3, second selector l) and a connector F. The first selector C is the switch whichoperates in response to thcoperation of the Substation calling device'for the first digit ofihe calledjnumlie a l-he. first selector not pern'ianently connected with the calling line, but 'the. sub scriber obtains connection thereto by means" of. the abovementionlal preliminarfitrunlv selecting switch Ii. whicll is individual to the subscribefis line and which is operated immediately upon the removal of his receiver. Said switches j are arranged in groups, each group-having comntdn access toa given number 0 trunk lines, usually ten which lead directly to first selectors. I have. iunvevcr. shown herein a system in which some of the trunk lines leading'to first selectors are connnim to a plurality of groups of line switches. and certain other trunk lines are individual to a singlegroup. For example, in Fig. -l there ane represented two 'groups of trunk tern'iinals N and L, each group belonging to a separate group of line switcl-ics; Some of the trunks, in this case the first and last, in each of thegroups N and L lead to individual first s'electors,'while the remainingtrunks are multipled between the two groups. so that two terminals, one

trunk line and therefore to a single selectOI'lI. l

The automaticsubstation shown herein may be of. any suitable or approved-type of automatic. substation. Referring especially tosnbstatibn A, it comprises the usual receiver 2; transmitter 3l,'svtit'ch' hook 4-, ringer 5 and condenser 6. Being tln-fllltOl'IltltlQ snbstation it is also providedwith an automatic calling device. The culling device '7 by a solenoid 12.

the 1naste1'- s\\-'itch there is a relay' 95 thev -may be of the type shown and described in British patent to S. G. S. Dicker No. 2%),(35i 4 of 1.910. As shown herein, however, the calling device is represented diagrammatically by a pair of impulse springs T and 8 and a toothed impulse wheel 9. The latter maybe controlled by the subscriber through the medium of a dial (not shown) provided with finger holes in the well-known manner. The .preselecting or line switches referred to above, as is well known, are each provided with a plunger which, when idle, is always maintained opposite the bank terminal of the idle t'runk' by a so-called master switch, The master switches Hand E control plungers of separate groups of line. switches, the line switch B l'ielonging to the group which is controlled by the master switch E. The.- same explanationwill apply to the line switch B and'its relation to the master switch E. The said masterswitch is sommvhat different from that shown in said British Patent N01.

1421 of 1910,being similar to that shown in French Patent No. 9,234 issued to the Autolnatio Electric Companyr As shown in Fig. 1, the usual plunger guideshaft- .10", through the medium of which the master switch controls the plunger-s, is connected with the rotary shaft 10 in such a manner that as the shaft 10 rotates back and forth the plunger guide shaft 10 is caused to oscillate about said shaft 10 as an The shaft lOxis adapted to be rotated in one direction by a spring 11 and in the other direction against the opposition of said spring Secured to the shaft 10'v there is a cam member 14: provided with notches lifl, one for each trunk line towhich the line-switches have access. The shaft is normally locked against rotation in either direction by the engagement of the lever 15 with one of the notches 13'. The lever 15 is controlled b the armature of the relay 56 in such a manner that when said relay is energized the .lever is withdrawn from the cam soas to unlock the shaft 10, The speed of rotation of the sliaftlO, and necessarily the plunger guide shaft 10, is controlled by a suitable governor 17. Associated with springs of which, upon being brought together, are locked in their operated posi tions hy" tl1e engagement of the spring 19 with the detent on the under side of the spring 20. The springs are unlocked by the arm 21 when the shaftlO reaches'the limit of its travel in one direction. When-the shaft 10'- reaches the limit of its travel in the other direction. the arm 22 forces the springQSLinto engagementwith the spring 24. As has heretofore been stated, the two master'switches E and E control separate groups of line. switch'plungers, andeach mas ter switch, unlike-the usual type, isp'rOVided with an uxiliary bankof QOIItaCtS- K -wniprising the common segment 25 and individual contacts a, in addition to the usual bank of contacts K comprising the common segment 25 and individual contacts a. Both wipers of said banks are. mounted on the same shaft and operate in synchronism. In general, the operation of the master switch is as follows:

Whenever a trunk line is seized by a line switch the relay 5G is energized to unlock the shaft 10, the spring 11 thereupon rotating-the shaft 10 to cause the plunger shaft to move the idleplungers away from the trunk just seized to a position opposite an idle trunk line. When the'idle trunk line a. reached the relay56 de'tinergizes and again locks the shaft. In the course of the operation of the system, when the plungersreacln a position opposite the last trunk line the arm 22 forces the springs 23 and 24 into contact, so that when this trunk is seized the cirfour of the relay 95 is closed. The relay 9?) thereupon energizes and the springs operated thereby become locked in their operated positions. This closes the circuit of the solenoid 12, which rotatesthe shaft 10 back in the opposite direction, bringing all the plungers which are in engagement with the plunger-shaft back to a. position opposite the first trunk line and restoring tension to the spring 11, When theplungers reach their starting positions the arm 21 unlocks the. springs of the relay 95, whichpermits the solenoid to denergiz'e and the plunger shaft to come to rest. The master switches E andv E are alike, correspondin parts being represented by the same re erence numerals. The contacts'of the banks K of the master switches E and E, with the exception of the first and last, are Wired in multiple. .The first and last contacts of the bank K'of the l'l'ltlStGllSWitGh E" are Wired to the springs 26 and 27, respectively, and the'first and last"- contacts of the bank K of. the master switch E to the springs 28 and 29, all ofwhich are. normally grounded at G.

cordance with the trunking diagram shown in Fig. 4. The wipers 30' and SOP-of the master switches E and ,E .keep'the common segments 25. and 25'in=.electrical connecti m with the individual. contacts win their re- I spective banks. corresponding to thetrunkline. before which the plnngersof the idle switches are resting It has already been stated that the first and last contacts of both banks K are" separately and temporarily grounded at G. Inaddition to this,;the re-.

I forewhich it is being held contact-in pa1rs, as shown i of. the springs (37 and 69 with the springs 68 is initiated the circuits tact betwecuathc springs "the" locking dog lfiout of engagement with "one of the notches I soon as the cam is unlocked the springll la s a2, 33,

be energized all the time the four terminals are grounded at G; In connection with the relays 32, 33,

chain relays, onlythree of which are shown in the accompanying drawings, which are wired, one by one, to each of theeight multiple trunk l nes, each being energized as the ground 'potential 1s placed on the contacts a of either of the master'switcii banks K in a a manner hereinafter described. WVhe'n a call tion with the master switches E and 111 are "as follows: a g

is well. known, the plunger of the line switch B of the calling line is normally held opposite the terminal of an idle trunk line. 20'

\Vhen the calling subscriber removes his receiver preparatory to making a call, the closure of the hook switch springs 60 and 1 completes abridge across the calling line through the transmitter 'and. receiver,

whereupon the line winding 62 of the switch B becomesenergmed.

The winding (32, upon energizing, attracts the armature (33, whereby the circuit of the operating winding (it is closed. The winding 64, upon energizin attracts the plunger arm 65 and the cut-off armature 66.v The armature 66 operates to disconnect the line winding 62 from the line, a while the plunger arm 65 causes the plunger to be thrust into the group of contact springs forming the terminal of the trunk line heand forces the sprin s 67, 68, '69, 70, 71, 72, and Z4 into The engagement and 7 0, respectively, extends the line circuit through to the first selector C, while the springs, 75 and 74- engage to automatically move Hie wipers 30 onto an idle trunk line in thetollmving manner:

WVhcn the springs 75 and .74. are -forced into contact by the plunger of the linesv .vitch, a circuit is closedtrom ground (i through said springs, master switch hank contact' 30, common segment and the 150, wiper lead 100. The relay 54,

relayv 54 to battery upon energizing, coi'npletes a circuit through theh'ela v 56, whereupon said relay e'fie rizes to separate the springs 80, close con- RL and withdraw 'or a 1.?) of the cam 14. As

5 causes the plunger" guide shaft 103 to oscill conicsfree from the guide "late and to move'the plungers of the idle line-switches away from the trunk line 111st seized lift" the switch B.

was the plunge-r of the switch B vengaging-with the trunk teru'iinal oppositiwhich resting. the switch I} beit has just been l shaft i0, and is 34 and are also wired to said first .and last contacts and to battery. It will therefore be seen that said relays will 34 and 35 there are eight other ,gizing of the relay 56 the 'lnto one of the notches 13, thereby locking involved in connec-- an idletrunk line also.

not afiected by itsmovemen t at this time. After the plungers have been moved a short I distance the master switch wiper 30 passes off the grounded contact .150, which also'allowsthe relay 54;.to deenergize and break the circuit of the relay Upon thedeenerdog 15 drops back the shaft 10' and bringing the plungers to rest in front of the next idle trunk line. There is also completed a circuit through the chain relay. 3? connected to the trunk line thathas just been engaged to battery. Upon said relay energizing, its armature moves the spring '40 over half way to meet the spring 39. ,Vhen the next trunk line is engaged the chain relay 36 connected therewith will also energize, whereby its armature is attracted and pushesthe springhi) over to meet and make contact with the spring 40. Thesame operation is repeated upon the engagement of each trunk, and should none of those previously engaged become idle in the meantime, therewould be a complete circuit from ground G through the armature springs 39, 40, etc., of the eight chain relays to the relay 42 and to battery. Upon the energization of the relay 42 ground battery is removed from the first and last contacts of both the banks K. Upon ground battery being removed from said contacts are allowed to remain upon the wipers 30 and engage with thepindividual trunk lines.

It is clearly seen that unless the eight multi '100 their respective master switches to remain in engagement'with said ple trunk lines are busy.it is impossible for the wipers 30 of individual trunks. The two wipers the master switches E and E are moved to gether while traveling over the eight multiple trunksthat is, the wipers of both mas- .ter switches rest on corresponding contacts.

Themeans for accomplishing this is' in the auxiliary banks K, the operation of which will be fully described hereinafter. Since the trunk line which is shownfas having been scivled by the line switch B one of the multiple trunk lines, it will therefore springs 75 and relay St and cause more its plungers to relay 54 of the master switch IE will. also be energized in the same manner, and the i'i'iaster switchli' will move 74' is closed to energizethe the master switch E to The manner in which the banks K insure that the wipers of the two master switches shall rest on corresponding contacts is as follows: The banks K have, as has already been stated, eight of their contacts multipled together, while the first and last contacts of the bank K of the master switch F. are grounded at (W'and (i and the first and last contacts of the bank K of, the

its plungers m be seen that when. the contact between the v an idle trunk line, the

master switch E have no connection whatever. Connected to the common segment 25" of the master switch E there is a relay 52,

which is of high resistance and is permanently connected to battery. 'This relay is the circuit of the starting relay 54 of the master switch E, it will be seen that said master switch will be unable to come to rest unless its wiper is in engagement with the contact which is engaged by the wiper 30 of the master switchE. I I

Assuming that the eight multiple trunk lines are busy 'and the wipers 30 of both master. switches are standing opposite corresponding idle individual trunks, and that.

a subscriber occupies one of these said trunks,.say of the master switch E,-theremainingidle plungersiwhich are controlled by this master switch only will rotate to 25* and the relay iKZ'tobattery; 100.

'in a manner.heretofore explained.

the eight ini'ilt'iple trunk lines become busy and the wiper 3.0 ot the master switch E find an idle trunk, which Willbe the other individual trunk line inthis particular group, leaving the plungers which. are controlled by the master switch. E stationary and'still opposite an idle individual trunk of said master switch E. \Vhen this trunk is eventually seized the idle plungers con trolled by the master switch E will likewise be automatically rotated onto the re maining individual trunk lines in, its group \Vhen is rotated onto one of the idle individual trunk contacts of the bank K, the relay 52 is held energized to keep the master sivitch from oscillating back and forth- For e'vf ample, if'the Wiper 30* passes onto the contact connected to ground G, the energizing circuit for the relay extends from ground (i through the wiper-'30, common segment Of course, it' the master switch stops on the contact'connected to ground (i the. energiz ingcircuit for the relay 52 extends from ground (1? instead of from ground G over the circuit just traced.

There are. now assumed to be engaged ten trunk lines, leavingxtwo still vacant. one in each group. ll hen one of these said trunks is engaged, say, for instance, the remaining trunk. in the group to -which the line switches B have access, all thecontactsa 47, which cuts off battery current from the master switch E, as well as from the op-- 'erative w ndings of the line switches B, the

circuit being as follows: from. ground G through the armature contacts of the eight chain relays 36, 37, 38, etc.,through the armature contacts of the individual chain relays 32 and 33, and relay 47 to the battery lead 100. Upon thelrelay 47 energizing the contact at the spring 48 is broken, at which time the circuits of the relay 56and the 0p-v erative windings of the line switches B are oper'ied. Upon thefailure of the relay -56 to operate the lever 15 remains locked in engagement with the cam 14, thereby pre venting the plunger guide shaft'from oscillating in the usual manner. and arrestingall attempts to operate said switches.

lhen the'reinaining individual trunk line of he group to which the --line switches B have access is engaged, the master switch E. operates in the same manner as described in connection 'withthe masteriswitch E. The circuit in this instance is completed from ground G through the armature contacts of the eight chain relays 36, 37,; 38,. etc., througlrthe armature contact of the individual'chairi relays 34 and 35 t0 the relay 47 of the master switch E and to battery.

. The line switch B and the. substation A are in all respects the Same as the line switch P) and the substationA.

At H (I ig.;-3) there is shown a rin ing' current generator and at I a busy signa ing device isrepresente'd. At J there is shown a battery having one tei'minal grounded at G for supplying current for operating the central ofiice apparatus and fomtalking purposes. I

Having givema general description of the aparatus. it will nowbe explained how the I subscriber of the calling"substation A may obtain connection with thesubscriber of the callcdsubstation Aflthe number of which.

latter will be assumed to be'2226.

' It has already been statcdthat. wheii -a.

springs 0 and 61 completes a bridge hcross the calling line through'the transmitter and receiver, whereu ion the-calling subscribei"s line switch plunger operates toengage with. the-idle trunk line. before which it is being held, and forces the springs 67 -68; 69-70, 7172, and 75-74. into contact in The engagement. of the spri ngs 7 and '69 with the springs (58 and 70, respectively. extends the line circuit. through to the first selector (l.

The line; relay 73 of the switch C thereby becomes energiaedhnd closes the circuit of the relay E6. The relay 7 6, upon energizing,

subscriber removes his receiver prepmratoey 1 1 5 to making a rall'the closure ofthe liook' completes a circuit from ground G through the springs 77, conductor 7S, sprin s 71 and 72 and the holding winding 49 of t 1e switch B. When the bank springs-T5 and 74 of the switch B are for'ced into contact by the operation of said switch, circuits are invol'ved which operate the master'switches E and E, which have already been explained. The line 18, therefore, now extended to the first selector C and the calling subscriber operates his-calling device in the well-lmown. manner for the SHK'CBSSlVtEllIQltS of the num' ber of the called line; The switch responds to theimpulses for the first digitLthe Second selectorl) responds to the impulses for the second digit and the connector F responds to the iliipulses for the last two digits to extend the connection toward the called line. 7

The operation of these swltches is, 111 general, we

19L0,,hereinbefore referred to. and it is not thought necessary to describe the same in detail herein.

After a conversation is completed the enti-re connection is autonmtically released by the hanging upof the receiverat the'calling substation. This is brought about in the following manner: During the time the conversation is being carried ou'thc release relays 97, 98 and TGof the switches F. l)

, and G are maintained energized by a circuit store the switches to normal position. The relay 7(5 of the. selector upon dei nerglz i a phone .of their respective switches.

ing, also destroys the holding circuits of the winding T9 of the switch I). whereupon the plunger of said switch is permitted to be restored to. normal position and all the. ap-

pa ratus employedin the connection is left another call.

illustrated my invention in connection with one particular type. oi telesystem and in conucctil'm with switches of a partimllar ty 'ic. it to-be understooi-l. of course. that my invention is of in readiness for While I ha .ve

broader scope and that I do not wish to belimited to such detailedconslructimis. Fur- -therniorc, while l have shown trunk lines extending from the line switches divided into groups of ten. two of which are individual to a group.whilceight are common, it is..orfc.om-se. understom l that this is merely an arbitrary selection and that the number oi" individual and common trunks can lay varied to suit. the working conditions.

I understood in the art and is fully. described 'in British Patent No. llzlt ot' \Vhat I claim as my invention is: V

1. In a telephone system,.a lurality of groups of trunk lines, means inc udi-n master switches operating over said trun lines to automatically select, an idle one of said trunks, and means whereby a pluralityof said master switchesmay be operated in synchronism. I g

2. In a telephone system, groups of sub: scribers lines. trunk lines individual to said groups of subscribers line's. other trunk lines;

common to a plurality of groups of subscribers lines, means associated with each group'of lines for automatially-selecting dle trunks. and means for causing a plural ty of said selecting means to operate in synchronism.

3. In a'telephone system, groups of subscribers lines, a plurality 'of trunk lines. certain of said trunk lines being individual to each of said groups of subscribers lines, I

while other of said trunks are. common to a plurality of groups. means for selecting, an

idle trunk. and means for preventingone of said trunks individual to a group being selected until all of the common trunks for this group are in use.

4. In a telephone system. a plurality of groups of subscribers lines, trunking switches individual to said lines for selecting idle trunk lines. trunk lines common to all of said groups, trunk lines individual 'to each of said groups. and means for preventing'the selection of. an individuiil trunk whenever one of the .coinmon trunks is idle.

5. In a telephone system. two groups of subscribers lines. amaster switch and a group of trunking switches foueaclrgroup of lines; a bank ot'contacts and a wiperfor each master switch. a relay adapted when deiinerglzed to close the starting cu'cuitof one of the. inaster switches. and an energiz-- mg circuit for said relay including corresponding contacts of both banks.

u l i) (i. ,In a telephone system. a plurality. of groups of "llllGS. trunking switches foinsaid lines divided into corresponding groups.

trunks divided into groups individual to each of the groups ofswitchcs. trunks-com:

"inbn to all of the .glOlll'JS. and means for maintaining the individual trunks of a.

'trunks for said trunking. switches, saidgroup inaccessible .to its switches as long ajs any of the, common trunks are idle.

7. In a p, telephone system, .a plurality of groups of lines. trunking switches for said lines divided into corresponding -groups, trunks for said trunking switches, said trunks divided into groups individual to each of the groups of switches, trunkscommon to. all ar the. groups means for maintaining the individualtrunks of a group inaccessible to its switches as lo as any of the cmnnnion trunks are idle, am means for making the llldljltltlfl-l trunks of. a group sible to the switches of sa d trunks when all of the common trunks are busy, means for. rendering switches of a group inoperative when all the individual and all of theQc0m-.

one of the individual trunks of that group becomeidle, and means for restoring to 01)- erative condition switches of said group andswitchcs of any other group rendered inoperative for similar reasons it any common trunk becomes idle while such switches are insuch inoperative condition. y

9. In atelephone system, trunking switches divided into groups, a master switch forcon- "trolling the switches in each group, trunks common to said groups of trunking switches, means associated with said ma'ster switches for enabling them to preselect for the trunking switches the trunk next available for use means for enabling a trunking switch H1 seize the trunk thus'designated and to then initiate the operation of its master switch in preselecting the next trunk,- means for initiating the operation of the master switch of anothergroup, and means for continuing the operation of said second master switch until it reaches a position for des-i nating the same trunk designated by the rstmaster switch.

' pair of other group 0 10. Ina telephone trunkin system, a pair of grou s of trunking switc ges,'a plurality of trun s common to ,the switches of said groups for trunking purposes, trunk select ing means for each group, trunks individualto each group, and means controlled by the group'of common trunksfor making the other roup'of trunks busy.

11. n a telephone trunkin system, a pair of groups of trunking-switc es, a pluralityi of trunks common to the switches of said groups for trunking purposes, trunk select-. ing means for each group, trunks individual to each group, and means controlled by the group-of co'mm"n trunks for making the p run'ks busy, busy test circuits for some of said.trunks,said la st means controlled over said busy test circuits.

12. In a telephone trunking system, a: groups of trunking switches, a plurality 0 trunks common to the switches of said groups for trunking purposes, trunk se- 'vidua to each by, the group o of trunks common-to the switches of said groups for trunking purposes, trunk selecting means for each group, trunks individual to each group, means controlled by the group of common trunks for making the other group of trunks busy, a busytest circuit for one of said common trunks, said last means controlled through the medium of said busy ji non \trunks are busy, means for restoring 7 such switches to operative condition if any test circuit.

14. In a telephone trunking system,- a pair of groups of trunking switches, a plurality groups for trunking purposes, trunk select ing means for each group common to the group, trunks individual to each group, Y

means controlled by thegroup of common trunks for making the other group busy, and busy test circuits .for some of said trunks, said last means controlled over said busy test circuits. r

16. Ina telephone trunking system, a'pair of groups of trunking switches, a pluralityof trunks common to the switches of said groups for trunking purposes, trunk select Lug means for each group common to the group, trunks individual. to eachgroup,

means controlled by the group of common trunks for making theotheiflgrou'p busy, and busy test circuits fOI said common trunks, said last means controlled through the medium of said busy test circuits.

17; In a telephone trunkin system, a pair I of groups of trunking switc es, a 'plurality' .of trunks common to the switches of said' groups for trunking purposes, trunk select;

in'g means for each group common to the group, trunks individual to each group, means controlled by the group of common trunks for making the other group busy, and a busy testcircuit for one of said common trunks, said last means controlled through the medium of said busy test cir: cuit. v

18. In atelephone trunking system, a pair of groups of trunking switches, a plurality of trunks common to the switches o'f said.

groups for trunking purposes, trunk selecting means for each group comprising a com-.., 13O

mon element in each group for assignin to the switches an idle trunk, trunks indivi ual to each roup, and means controlled by the of switches for trunking,

group o common other roup of trunks busy.

' 19. :1 a telephone trunking system, a pair of roups of trunking'switches, trunks indivi ual to one of said groups of switches for trunking, and trunks common to both groups trunks for making the means for each group, and means for mak ing the individual trunks inaccessible to the switches, said means under the control of trunking, and trunks .of switches for trunking, trunk se ecting' 20. I

' L individual .fswitches, said means under potential to one of the trun said common group.

20. In a telephone trunking system, a pair of groups of trunking switches, trunks inclividual-to one of said groups of switches for common to both means for each group,

means for making the trunks inaccessible to the, the control of, saidcommon group, a bus each trunk, means for app ying a busy test of one vof sai ,u'hen seized by one of said switches M e the seized trunkbusy, and means trunk selecting 'oups I test circuit for Witnesses:

under the control of all of the trunks of the other group for applying a busy test potential to the same trunk.

21. In a telephone trunking systel'n, a pair of groups of trunking switches, trunks individual to one of said groups of switches for trunking,

groups of switches for, trunking, trunk lectmg means for each group, means for making the individual trunks imiccessible to of said common group a busy test circuit for each trunk, means for applying a busy test potential. to one of the trunks of one of said groups when seized by one of said and trunks common to both I "the switches, said means under the control switches to make the seized trunk busy, and

oyher means for applying a busy test poten- H. A. SWENAR'LDN', J. NounY.

- 1 (look county, 

